| Abstract: | The role of endogenous opioids in the mediation of stress-induced analgesia has been studied using the opiate antagonist naloxone to reduce or eliminate the response. While the analgesic responses following some stressors are reduced by naloxone, other stressors, like cold-water swims, are altered minimally. However, in the case of inescapable foot shock analgesia, the temporal, numerical, and spatial arrangement of the shocks are critical parameters in determining whether naloxone is capable of altering the analgesic state. In assessing parametric variations of naloxone antagonism of cold-water swim analgesia, five experiments were performed. The first experiment showed that naloxone antagonized the analgesic response following a 3.5-min swim in a 15°C bath, but not in baths of 8°C and 2°C. The second experiment demonstrated dose-dependent antagonism of analgesia induced by 2°C swims for 2.5 and 3.5 min; shorter durations failed to increase thresholds. The third experiment indicated that naloxone decreased 2°C, 3.5-min swim analgesia when the pain test occurred 30 min after stress; longer intervals failed to produce analgesia. The fourth experiment showed that the temporal relationship between injections and swims had little bearing on resultant effects. Finally, since it appeared that naloxone decreased analgesia induced by the 2°C, 3.5-min swim in some animals, but not others, the fifth experiment found that the degree of naloxone antagonism was correlated with the magnitude of the analgesic response induced in individual animals. These results are discussed in terms of opioid and nonopioid mechanisms subserving pain inhibition. |
